Simply increasing wavelength multiplexing numbers and laying new optical transmission networks is insufficient in terms of meeting the demands involved in realizing large-capacity optical transmission. Accordingly, transmission speeds have increased from conventional 10 Gbps to 40 Gbps and 100 Gbps (see for instance Patent Literature 1 and 2).
With increasing speeds, however, transmission penalties such as distortion of optical signal waveforms caused by wavelength dispersion or polarization mode dispersion in the optical fiber of the communication channel become serious, in particular in optical transmission systems where long-distance transmission is carried out.
In order to solve the above problem, digital coherent communication schemes are resorted to that allow compensating wavelength dispersion and polarization mode dispersion in the electrical domain, and that allow supporting various multilevel modulation schemes, by providing a digital signal processor (DSP) on the reception side.
Multilevel modulation such as DP-QPSK (Dual Polarization Quadrature Phase-Shift Keying) modulation and DP-BPSK (Dual Polarization Binary Phase-Shift Keying) modulation, which are digital coherent schemes, has been used in recent years.
Other known schemes involve doubling the number of transmitted bits per symbol, relying on a polarization multiplexing scheme, in order to realize wavelength multiplexing of higher density that enables long-distance transmission. In polarization multiplexing schemes, independent transmission signals are respectively allocated to two orthogonal polarization components.